Resonator for oscillators



y 1946- D. H. SLOAN 2,404,542

RESONATOR FOR OSCILLATORS Original Filed Nov. 4, 1940 10 Sheets-Sheet 2 !/7 1, 712 //4/ I II/ 2 g a! 1 a T Zo- INVENTOR.

DAV/0 H. SLOAN.

By M ATTORNEYS.

July 23, 1946, D. H. SLOAN RESONATOR FOR OSCILLATORS Qriginal Filed Nov. 4, 1940 10 Sheets-Sheet 4 IN VE N TOR.

DA V/D H. .SL 0A N.

A TTORNEYS.

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\L' I \\\\\I y 1946. D. H. SLOAN RESONATOR FOR OSCILLATORS Original Filed Nov. 4, 1940 10 Sheets-Sheet 5 IN l/E N TOR.

0A V/D H. SLOAN.

AT TORNE YS.

July 23, 1946. D. H. SLOAN I 2,404,542

RESONATOR FOR OSCILLATORS- Original Filed Nqv. 4, 1940 10 Sheets-Sheet 6 INVENTOR.

DA we H. SLOAN.

ATTORNEYS.

y 1945- D. H. SLOAN 2,404,542

RESONATOR FOR OSCILLATORS Original Filed Nov. 4, 1940 10 Sheets-Sheet 7 fig .22.

INVENTOR.

DAV/D H. SLOAN.

A TTORNE'YS.

July 23, 1946. D. H. SLOAN 2,404,542

RESONATOR FOR OSCILLATORS Original Filed Nov. 4, 1940 10 Sheets-Sheet 8 INVENTOR.

DAV/D H. SLOAN.

, ATTORNEYS.

July 23, 1946. o. H. SLOAN RESONATOR FOR OSCILLATOBS Original Filed Nov. 4, 1940 l0 Sheets-Sheet 9 IN VE N TOR.

DAV/D h. SLOAN.

BY QM? I ATTORNEYS.

July 23, 1946. D. H. SLOAN 2,404,542

RESONATOR FOR OSCILLATORS I Original Filed Nov. 4, 1940 10 SheetS Sheet l0 jigga F ig .30.

INVEN TOR.

DA v10 H. SLOAN.

A 7' TORNEYS.

Patented July 23, 1946 UNITED s'rA'rss PATENT oer-flog 1,404,542 a RESONATOR FOB OSCILLATOBS David H. Sloan, Berkeley, Calif" aasignor to Research Corporation, New York, N. Y., a corporation of New York Original application November 4, 1940, Serial l lo.

864,284. Divided an 21 Claims;

d this application June 9, 1941, Serial No. 897,235

This invention relates to electronic tubes, and

particularly to tubes adapted for the production and modulation of ultra-high frequency oscillations, 1. e., oscillations of frequencies of the order of 1,000 megacycles. This application is a division of my prior application Serial No. 364,284, filed November 4. 1940.

The progress of electronic and radio development since the inception of the art has been marked by two steady advances. One of these advances has been toward higher power, the other toward higher frequencies. The latter line of advancement has been, to a certain extent at least, incompatible with the first, since with increasing frequency the effect of interelectrode capacity has become greater and more troublesome. Nevertheless, up to the last few years, the difiiculties have been met by a steady evolutionary process consisting in large degree of refinement in detail, which has enabled the vacuum tube art to keep pace with the increasingly rigid demands of the manufacturers and operators of transmitting and receiving apparatus.

The attempt within recent years to carry the useful spectrum into the range of wavelengths in the range of a meter and less has involved dimculties of a new order of magnitude. For one thing, the frequencies involved are so high that the transit time of an electron stream across the interelectrode spaces of the tubes becomes an appreciable fraction of a cycle., For another, even with connecting leads reduced to minimum lengths, their inductance has been suflicient so that the capacities required in tuning them to the desired frequencies are small in comparison with the interelectrode capacities in conventional tube structures and these capacities have therefore become not merely a nuisance, limiting the efficiency of operation, but frequently an absolute bar to such operation; so much so, in fact, that it has been only with tubes of very small size and consequent small power output that operation has been obtainable at all. 1

There therefore exists at the present time a need for a tube which will meet the severe requirements of producing large power outputs by generation or amplification at extremely high frequencies. These requirements are first, a cathode-grid structure which will effectively modulate an electron stream without the application of excessive control voltages; second, a cathode-grid structure whose capacity and inductance relationshipsare so proportioned that they may be tuned to the high operating frequencies desired; i

third, a structure lending itself to circuits of low relative radio-frequency resistance of high impedance, so that excessive energy will not be required to swing them through the necessary range of control voltages; fourth, fixed relationship between the various elements, irrespective of temperature or ordinary shock. so that the frequency to which the device as a whole is tuned will not be affected by relative changes of position; fifth, minimum undesired or "incidental" radiation from the various elements of the tube and its auxiliaries; sixth, minimum of insulating material subjected to high-frequency fields. To these may be added the secondary requirements of demountability for replacement of filaments, facility in water cooling and avoidance of hot spots, and ease of tuning. From the conventional approach these requirements are incompatible to a large degree. A high degree of control requires close spacing of cathode and grid, which leads to high interelectrode capacity. Rigid structure ordinarily means massive structure, which again leads to high interelectrode capacity. Water cooling systems tend to form effective antennae, leading to large stray power radiation. The broad purpose of my invention is therefore to reconcile these and other apparent incompatibles. 7

Pursuant to this generalpurpose, among the objects of this invention are: to provide a tube which is capable of producing many kilowatts of power at extremely high frequencies; to produce a high frequency generator of great frequency stability; to produce a high frequency amplifier and oscillator tube of relatively high efllciency, and particularly to produce such a tube wherein the losses due to undesired radiation from the tube itself are reduced to negligible proportions; to produce a high frequency oscillator and amplifier which may be tuned to operate at any.

desired frequency throughout a reasonably wide range; to provide a high frequency oscillator and amplifier which may be constructed with the high degree of accuracy required to meet the close tolerances demanded by the frequency of operation and maintain those tolerances under the changes of temperature produced by such operation; to provide an electronic tube of the character described which may be fully fluid cooled and wherein the cooling system does not introduce material parasitic radiation of radio-frequency power; to provide. a high power oscillator and amplifier tube which is readily demountable for replacement of filaments; to provide means of density-modulating an electron stream at ultrahigh frequencies, in order to produce extremely short bursts of electron emission occurring at peaks of the oscillation and of substantially uniform velocity, whereby the conversion of energy transmission line of at least one and 3 ratus of the character described; and to provide a type of electrode support for high frequency electronic devices which is-massive and rugged. and which, attlie same time. does not introduce interelectrode' capacities which either severely limit the frequencies upon which the device is oper'ative or the power which may be developed at such frequencies.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing. will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that 1 do not limit myself in any wayto the apparatus of the present application, as I may adopt various other apparatus embodiments utilizing the method. within the scope of the appended claims.

The tube of my invention involves two basic 2o concepts. The first of these comprises forming electrode supports of sturdy coaxial metallic cylinders which constitute a radio-frequency preferably a plurality of quarter wavelength electrical links, 5 with impedance irregularities at or near certain of the quarter-wave points, the electrodes themselves forming a portion of these transmission lines as considered electrically. Means are preferably provided for varying the position of the impedance irregularities to provide exact tuning, but this is not essential since. as will hereafter be shown. by a proper combination of the characteristic impedances of the quarterwave sections and their terminating impedances, j it i possible to make the radio-frequency impedance of the supports as viewed from the electrodes themselves extremely high, so that the overall effect is almost as though the electrode capacities together with the inductances required i to resonate them were supported freely in the 1 space within an unbroken metallic shield. This latter feature is secured by providing multiple coaxial line sections forming greatly different impedance, certain of these 4 paths acting as'by-passes of ance at points where it is necessary that some circulating currents should flow, insulation must be maintained, time maintaining the high impedance desired in other paths which would otherwise lead to radiation. By placing these by-pass sections at branch paths of negligible impedalthough D. C. while at the same lcurrent nodes. the PR losses therein may be made too small to need consideration.

The'second fundamental concept comprises mounting on the ends of such supports, preferably in biaxially symmertical configuration, one or a plurality of cathode-grid combinations which act, as before stated, as the termini of the transmission lines formed by the supports; mounting the grid opposed to an anode or other accelerating electrode in such manner as to produce an electrostatic field between grid and accelerator which comprises lines of force very sharply curved in the immediate neighborhood 6 of the grid; and mounting the cathode in the' region of sharpest curvature. One of the best ways of obtaining such a structure is to form the grid of pairs of cylindrical surfaces whose axes are parallel to the plane of the anode, and to make the cathode as a filament or strip having a fiat or slightly concave face lying between the cylindrical surfaces 01' the grid. This results in the lines of force from accelerator or anode nor: mally terminating in the grid structure, none of 7 structure.

them reaching the cathode. from which emission is therefore normally suppressed. A few volts relative change in potential of the cathode. "as referred to either accelerator or control grid,

results in some of the lines of force from the accelerator terminating in the cathode surface. which is accordingly subjected to an extremely powerful field causing very large emission to the anode. The result is what may be termed an "explosive" type of emission. giving electron bursts of high density for very short periods at the peak of the cycles. It will be evident that this structure results in a relatively high capacity as between cathode and grid. but the tuned transmission line support enables this capacity to be eii'ectively'resonated with an inductance as small as may be desired and still form a sharply tuned, high Q circuit whose high resonant input impedance may appear as resistive, capacitive or inductive. as the conditions of operation may require.

Referring to the drawings:

Fig. l is a longitudinal section through a high-frequency oscillator tube embodying my invention. the particular tube illustrated employing a radial arrangement of filaments and grids.

Fig. 2 is a transverse section of the tube of Fig. 1, showing the multiple coaxial grid-filament supports. and water connections for cooling the filament mounting, the plane of section being on the line 22 of Fig. 1.

Fig. 3 is a transverse section through the anode structure of the tube, the plane of section being indicated by the line 3-3 of Fig. 1.

Fig. 4 is an enlarged detailed view illustrating water-cooling connections from the exterior of the tube to the filament mounting.

Fig. 5 is a schematic sectional view through filaments, control grid. accelerating grid, boundary grid, and anode of the tube.

Fig. 6 is a sectional view through the grid support line, showing the radio-frequency by-pass between accelerating and boundary grids and the tuning mechanism for isolating the control grid and water cooling the same.

Fig. 7 is an enlarged detail showing the method of insulating certain of the supporting rings upon which the coaxial electrode elements are carried.

Fig. 8 is a section taken at right angles to the view of Fig.1, and showing the anode-supporting, cooling, and tuning system.

Fig. 9 is a perspective view of the accelerator grid.

Fig. 10 is an elevation of the control-grid Fig. 11 is a sectional view, taken on the plane between the filament and grid structures, and showing in detail the filament support.

Fig. 12 is a fragmentary axial section taken on the line i2i 2 of Fig. 11.

Fig. 13 is an elevation of the active face of the anode.

Fig. 14 is a fragmentary section of the anode, the plane of section being indicated by the line i4-i4' in the preceding figure.

Fig. 15 is an elevation of the boundary grid.

'Flg. 16 is a sectional View taken on the line i6 i6 of Fig. 15, and showing a portion of the anode in elevation.

Fig. 17 is an elevation of one of the filaments.

Fig. 18 shows a modified form of coaxial line structure for grid-filament support in a tube generally similar to Fig. 1, but adapted for use either Figs. 19.20. and 21 are'sectional views throush the tube of Fig. 18, taken on the lines numbered in accordance with the figures.

Fig. 22 is a longitudinal section through a tube built in accordance with this invention but wherein a cylindrical, rather than a radial filamentgrid and anod arrangement-is used. M

Figs. 23, 24, and 25 are transverse sections through the tube of Fig. 22. taken on the linesindicated by the respective numerals.

Figs. 28 and 27 are detailed views indicating the tuning mechanism for the tube of Fig. 22.

Fig. 28 is a longitudinal section on a larger scale through the filament support or the tube of g. 22. Fig. 29 is a transverse section through the supporting columns of the tube of Fig. 22, showing the construction of the centering mechanism, the plane of section being indicated by the line 28-48 of Fig. 28.

Fig. 30 is a sectional view taken on the line "-30 of Fig. 22.

Fig. 31 is a fragmentary section taken on the line !i8i of the preceding figure, showing the passage of the cooling pipe past the anode and between the two sections of the filament support.

Fig. 32 is an impedance diagram for an openended, half wavelength section of transmission line.

In the ensuing specification the invention will first be described in its various aspects as applied to an oscillator tube oi moderate power (1. e., approximately kw. peak output at 20 to 40 centimeters wavelength). Following this there will be described two modifications illustrating respectively the application of the principles of my invention to a similar tube adapted for amplification or for generation of oscillations by inductive feed-back, and to a somewhat higher output device showing the principles as applied to a tube constructed with cylindrical rather than radial arrangement of electrodes.

The tube shown in longitudinal section in'Fig. l is of the demountable, constantly pumped type, as in fact, are all of those herein described, although the principles involved are not limited for use in such tubes. From the structural point of view the tube comprises a series of flanges connected by sections of tubing and held together in compression. From a practical point of view it is advantageous to have the flanges pierced for andlield by circumferential bolts to hold the parts firmly in position when the tube is not under vacuum, but when in use the external air pressure tends to hold the entire device together, and the tube has actually been operated without the retaining bolts; these have therefore been omitted in the drawings since they add a further complexity of detail to an already complex structure.

Considering for the moment, therefore, only the external structure which forms the housing and which supports the remainder of the equip-. ment, the tube comprises an anode housing flange l which is grooved to receive tightly the end of a tubular anode housing 2. This housing fits an internal recess or counterbore in an annular grid flange 3, clamping a boundary grid 4 between the housing tube 2 and the flange. A rabbet on the outer periphery of the flange 3 receives one end of a main support cylinder 5, whose other end terminates in another annular flange I. All of the parts thus far mentioned are of metal, and I have found it convenient to make the flanges of steel, and the tube l also of seamless steel tubing, while the cylinder 2 may be either chromium or copper plated steel or'solid copper, with copper preferred. since it forms a portion of a resonating circuit. Carrying on from the flange 1 is a glass or "Pyrex" cylinder I which abuts a terminal flange l0.

As has been mentioned already, the device as a whole isfully demountable. The ends of the tubes contact the fianges with smooth machine fits. The joints thus formed are sealed by app ying thereto ordinary wide elastic bands as indicated by the reference characters ll, these bands being smeared before application with a small amount of vacuum line stop-cock grease.

It may be pointed out at this time that all or the structure thus far described with the exception of the terminal flange III is at D. C. ground potential, and as will later be shown in detail that the entire exterior structure is substantially at radio-frequency ground. This means that the insulating section formed by the cylinder 8 is not subjected to R. F. fields. It also renders easy the support of the device by any desired external means. Part of such support may be the connection to the pump, which is by a pipe l2 of relatively large lumen, welded or otherwise secured into the bottom of the anode housing 2. This p p is not shown in Fig. 1, but is clearly visible at the bottom of Fig. 8. v

The various elements which contribute to the electronic action of the tube are mounted within the envelope thus formed on columnar supports each of which has transmission line characteristics designed to meet its particular function. These elements are shown in schematic arrangement in Fig. 5, and comprise an anode it, a boundary grid 4, an accelerating grid IS, a control grid l6, and a filamentary cathode i'l.

Fig. 5 is drawn to a greatly enlarged scale and shows a fragmentary section of the elements co,- operating with a single filamentary cathode. In

the tube here shown six such cathodes are used.

and the portions shown of the other elements are repeated for each cathode. One advantage of the type of structure here shown is that the ability of the tube to supply power output varies almost directly as the number of filaments used, and that the changes required to add additional filaments are relatively minor. Tubes have been designed conforming substantially to the structure here shown with as high as twenty-four filaments, each with its attendant grid-anode structure, but since each of these assemblies is merely the duplicate of the others as far as performance is concerned, it is suiilcient for the present to consider one only.

Considering, therefore, the portion of the elements shown in Fig. 5, the anode i3, preferably made of high conductivity oxygen-free copper, is operated at the maximum potential of the system, say 10 to 50 thousand volts positive. It is provided with a V-shaped groove 20 with its axis parallelto the axis of the filament. Next, proceeding toward the filament, is the boundary grid 4, which is also preferably made of oxygenfree copper. This is provided with an aperture surrounded by a collar 2| in accurate alinement with the groove 20 in the anode. Next in line is the accelerator grid IS, with a slot 22 which is somewhat narrower than the opening in the boundary grid, and which isoperated at a potential above the cathode of from 5 to 20 thousand erating frequency'of the device. Furthermore,

modifications in design are possible whereby the functions of certain of the grids are combined. other electrodes are operated at ground potential, etc. Such modifications will be considered later; the purpose here is to show the application of the principles of my invention to the present tube. The most important portion of the combination is the arrangement of the cathode-grid structure. The important features here are first, that i the control-grid elements comprise parallel cylindrical surfaces, curved as they are presented to the filament. In the present case they are rods or wires. but they could be cylindrical surfaces formed as the edges of a slot in a fiat plate without affecting their performance. Between these surfaces, and slightly back of the plane of their centers of curvature, lies the filament, which has a. flat or preferably a slightly hollow ground face.

presented to the anode. It is convenient to operate the filament at ground potential (disregarding for the moment the slight voltage drop along the filament) and, for the powers here considered,

to operate the grid it at 200 to 500 volts negative.

It will be seen that at the orders of voltages given the major fields are from the accelerator grid i5 to the control grid. As is wellknown, the lines of force constituting such a field terminate at' right angles to the surfaces of the field- 1 defining electrodes. It follows that in theregion adjacent the-cathode the lines of force emerge from the grid wires in the general direction of l the cathode and then curve very sharply toward 1 the anode in a direction nearly at right angles i to their direction of emergence. There is also 1 a fairly strong field between the control grid and v the cathode itself, which is superimposed locally 1 upon the field between the control grid and acl celerator grid, and is directed toward, instead of away from the cathode. As a result of the interaction of these two fields none of the lines of force from the accelerator-grid normally terminate upon the surface of the cathode. Emission has therefore no tendency to leave the latter, 1 since the space adjacent it is nearly neutral, with such weak field as exists therein directed toward the cathode.

As is the case with any grid-controlled tube Ioperated through cut-off, when the grid swings positive some of the lines of force from the ac- Icelerator-grid which formerly terminated on the control grid now terminate on the cathode, and $3.5 the cycle progresses the cathode-contro1 grid lfield weakens or even reverses, permitting emission toward the anode, and a. space charge builds up in the region immediately in front of the oath. ode face which has the usual effect of limiting emission. The distinguishing feature here is that th region where the field is weak enough to permit such space charge effect is very shallow, so that even with the low velocities imparted to them by such relatively weak field the electrons can and do traverse it in a reasonably small fractio' of a cycle.

The biasing potential between cathode and grid is so adjusted that emission can occur only for an instant at the cycle peaks, and cut-oil may occur even before the first electrons emitted have traversed the space charge region. Furthermore,

while in this region there is a maximum difierence of velocity as between electrons, both by reason of differing initial velocities of emission and, more important, by reason of differing acceleration due both to phase of emission and field strength at various parts of the cathode surface.

through it before the cycle hasv advanced too far and, having traversed it. fall into the region of high potential gradient where acceleration toward the anode is very great; space charge effect is of no further moment, and they receive so large a portion of their final velocity that their differences of velocity .in the initial region are immaterial. f

It should be realized that while space-charge effect prevents some emission and decreases the acceleration of electrons emitted, it will not drive those which have been emitted back to the cathode nor prevent-their reaching the anode. It follows that the space-charge region may be consldered as a reservoir for emitted electrons. With conventional grid-cathode structures it is relatively deep, so that, at the frequencies and powers at which this tube is intended to operate, transit therethrough occupies a major portion of the cycle, and'with the varying velocities obtaining while in this region the electrons straggle through to reach the anode in such varying phases that the density modulation of the stream is almost if not entirely lost.

With the arrangement of my invention, however, the space-charge region is so shallow that even the stragglers among th emitted electrons traverse it in less than a quarter cycle and instead of the density modulation of the electrons being lost they reach the anode in bursts of such power and suddenness and with such close velocity grouping that I have termed cathodegrid combinations of this type explosive. The object of the design is to make the electron reservoir constituted by the space-charge region as shallow as possible, and in practice the ideal can be so. far realized as to permit density modulation of electron streams at frequencies in the range of 1,500 megacycles, where in th past it has been necessary to use velocity modulation, involving larger and more complicated structures, to get reasonably effective results, even in smaller sizes and at much lower powers than those here contemplated. v

When the tube here shown is used as an oscillator in the manner now to be described, the various potentials are 50 arranged and proportioned that the transit time of the burst of electrons is substantially one-half cycle. The anode i3 is in a tuned circuit, as is also the grid IS. The condition of oscillation then is that the potentials of the anode and the grid swing in the same sense, 50 that the grid reaches its peak of positive potential at the same instant as does the anode.

One of the results of the conformation of the electrostatic fields is a strong focusing action upon the electron bursts, and these bursts accordingly fall upon an extremely limited portion of the anode surface, substantially none reaching either of the intermediate grids. Th anode area upon which the electron bursts impinge is that included in the /-shaped slot 20. The reason for this arrangement is to spread the area of impact and so increase the size and decrease the intensity of maximum local heating, while increasing the cross-sectional area of thermal conduc- 'of the electron burst is one-half cycle of oscillation, and it follows that immediately after the electron burst has occurred the anode has started.

pass the effective plane of the boundary grid 4.

As the anode continues to swing negative they encounter a decelerating field, either in an absolute sense or, if still being accelerated by the D.-C. field, from the anode at least in comparison to the acceleration of the D.-C. field alone. In passing through this decelerating field the electrons are delivering ener y to the anode circuit, and they are traveling at minimum relative velocity when they enter the slot 20. This slot acts in some degree as a Faraday space, and the electrons suffer little change in velocity or energy as they pass through it. Therefore their work is done and their transit time effectively over as soon as they have entered it.

Since the acceleration of the entir burst of electrons takes place with substantial uniformity they retain their close grouping at the time of impact, and since the impact occurs when the it must be capable of being connected in circuit with an inductance sufliciently small to tune to that frequency. The accelerator grid must be insulated from the other elements to maintain its D.-C. voltage, but should be efiectively grounded as regards radio-frequency potentials, The boundary grid 4 should also be grounded to radio-frequency and for convenience in operation and safetys sake should preferably also be grounded as regards D.-C. potential, as it is electrically continuous with the envelope. The anode should be free as regards both A.-C. and D.-C. potentials.

The various mounting and auxiliary means next to be described are designed to meet the desiderata as fully as possible. In this description terms such as above or below are used to. indicate position as shown in Fig. 1. They have no other significance, as the device may be operated in any position.

Starting at the bottom of Fig. 1, with the flange III, a high conductivity column or pipe 25 extends a major portion of the length of the entire device to the plane of the flange 3 and the boundary grid 4. This column is brazed or otherwise permanently secured into the flange l0 so as to be accurately concentric with. the remainder of the tube structure and, of course, to be vacuum tight.

At its upper end it is threaded to receive a grid: support ring 21, which is clamped between locking nuts 29 and 30, and an additional locking screw 3| (Fig. is also provided for further security, The pairs of parallel grid wires l5 project from the ring 21 parallel to its radii, six pairsot grid wires being provided in the present design, the pairs being equidistantly spaced around the periphery 0f the ring. 7

Two sliders aremountedon the column 25. The upper slider 23 comprises a short section of 10 tubing 32 surfaced to a sliding fit on the column 25 and shouldered. at each end to receive discs 33 and 34 between which a short section of tubing 35 is clamped. The column 25 is provided in this region with a longitudinal slot for the passage of a screw 31 which engages a piece of tubing 33 sliding within the column. The tubing 33 terminates in an annular block 40; and an adjusting rod 4| is threaded into one side of the block and passes to the exterior of the tube through a gland box 42 and a Wilson seal 43. It is apparent that the position of the slider may be adjusted by sliding the rod 4|,

A word as to the Wilson seal may here be in order, and in this connection attention is drawn to the showing at the lower right of Fig. 8. The seal proper consists of a normally flat washer 45 of synthetic or natural rubber, which is forced against a conical seat 41 by the internally conical edges of a gland 43. When the washer is unstressed the aperture therethrough is slightly too small for the rod 50'which it is desired to seal. The seal is lubricated with a small quantity of vacuum stop-cock grease. Such a seal is vacuum tight under conditions where other known types of packing would leak badly, since the difierential pressure on the washer serves to make it hug the central rod more tightly and it remains tight whether the rod be subjected to rotary or sliding motion in either direction.

Returning to the general tube structure, the second and lower slider 5| is essentially similar in construtcion to that just described, except that its actuating rod 52 is mounted externally of the column 25 through the gland'box 42 and Wilson seal 53.

The'slider 5| makes a close sliding fit within a cylindrical conductor 54 mounted in the flange l0 accurately concentric with the column 25 and maintained in this concentric relation both by the slider 5| itself and by an auxiliary diaphragm or spacer'55. The tubing 54 does not extend the full length of the central column, but terminates a distance above the flange I which is somewherein the neighborhood of one-eighth of a wavelength at the mean frequency for which the tube is designed.

Accurately coaxial with the column 25 and its surrounding conductor 5| is a third conducting cylinder 51, mounted on the fiange I and extending below it for approximately one-eighth wavelength, so that the two conductors 54 and 51 overlap by a distance approximately equal to onequarter wavelength of the average operating freguency of the device, wavelengths in this sense 'being used-to mean the wavelength of the frepipe are brought out through the flange I at the right of Fig. 1.

The upper end of the column 51 carries an intermediate ring 6| which supports indirectly one end of each of the filaments H. The other ends of these filaments are carried by a group (here six) of pipes.62 mounted in the annular interspace between the column 51 and the outer shell 1'1 6. .The lower ends of the pipes 62 are mounted in a ring 63 which is bolted to and insulated from the flange 1 as is shown in Figs. 2 and 7. The ring 63 is counterbored at three equidistant points to receive insulating beads 64 of porcelain, lava, or other refractory insulating material which beads space the rings slightly from the flange 1. A cap screw 66 passes in turn through a clamping cap 61, a second bead 69, the ring 63 and the bead 64 to clamp the ring firmly to the flange. It should .be noted that the potentials which this arrangement must withstand are of low frequency and are only those across the filament, 1. e., the insulation need only be of sumcient value to withstand a few volts (three at 60 cycles in the instant structure) and the insulating material is not subject to dielectric heating from radio-frequency fields.

The actual'filament mounting can best be seen in Figs. 11 and12. Each of the tubes 62 carries an inwardly projecting L-shaped lug 10, and the inner ends of the lugs are provided with slOts 1| for receiving the downturned ends of the stapleshaped filaments l1, the ends being clamped into place by set-screws 12. The inner ends of the filaments are clamped in an annular groove 13, formed in an inner mounting ring 14 which is supported on column 51 by the intermediate ring 6| before mentioned. The actual clamping of the filament ends is accomplished by pairs of setscrews 15 bearing on small blocks 11.

The pipes 62 are surrounded by open-ended cylindrical conductors 19, which terminate at the level of the upper end of the lug and extend down over the pipe 62 for approximately one-quarter wavelength and are supported by the ring 6|.

Within the conductors 19 are inner tubular con-' ductors 80 of substantially the same length, open at their upper ends and mounted by their lower ends on the pipes 62 by means of conductive blocks 8|. The concentric tubes 18 and 80 are both open at the filament end, being notched to' clear the lugs10 and also being provided with aligned holes to permit tightening of the setscrews 12. It will thus be seen that the only connection between the inner column 51 with its supporting rings 6| and 14 and the group of filament support tubes 62 is the filaments themselves.

These are shown in Fig. 1'7, and as will be seen are relatively short and rigid. They are preferably of pure tungsten and have a considerable degree of strength. It will further be seen that the support afforded their outer ends by the tubes 62 and lugs 10 is light and of small inertia and that the tubes 62 have relatively large resiliency. The filaments therefore are very unlikely to be ruptured by shock on the device as a whole, and there is ample flexibility to take up their expansion.

Each filament is preferably formed of round tungsten wire, one surface of which is ground flat or slightly concave. The diameter here used is 50 mils. The grinding is preferably performed in a jig which deforms the wire slightly in the longitudinal direction, so that the ends of the filament are around a few thousandths of an inch thinner than is the central portion. This grinding forms the fiat emitting surface of the filament, and if done with a relatively. small wheel whose axis is maintained parallel to the length of the filament, it gives the slight hollow grinding which has already been stated to be advantageous. The effect of thinning the two ends, adjacent the point where the filament is clamped, is to give a greater current density at 12 these points, with a consequent greater liberation of heat which compensates for the heat conduction to the clamping means and results in substantially constant temperature and substantially constant emission over the entire effective length of the filament. ,Being of pure tungsten, the filament retains a degree of resiliency even at its emitting temperature, and this, together with its resilient support, prevents buckling or change of plane of the emitting surface when in operation and keeps the electrical constants of the device fixed under such minor variations in operating temperature and expansion, and in equality in these factors as between the several filaments, as inevitably occur in practice.

Cooling for the support of the inner ends of the filaments is accomplished by conduction through the support rings 14 and 6| to the column 51 and thence through the cooling coil 60. Cooling for the outer supports is by circulatory system within the support pipes 62 themselves. A small water pipe enters the side of each of the support pipes 62, and extends axially within it to a point adjacent the lug 10, so that water entering this pipe will be squirted against the inner end of the lug. From there it returns through the pipe 62 externally of the pipe 90 to the bottom of pipe 62, where the end 90 of the next pipe is connected to carry the water to the next filament support, circulation thereby occurring through each of the support pipes 62 in succession.

The supply for this circulatory system is through a fitting designated by the general reference character 9|, comprising coaxial pipes 92 and 03 which connect respectively to the two ends of the system. The outer of these pipes is permanently secured to the support ring 63 (see Fig. 4). The fitting 9| passes through the flange 1 and is insulated'therefrom by insulating bushings 94 of steatite or other refractory between which is a compressed rubber washer 95 forming a. vacuum-tight seal. A connecting lug 91 for connecting one filament supply lead is mounted upon the fitting ill, and the ring 63, and the circulatory system comprising pipes 90 and 62 all constitute the conducting system for supplying the filament current. The return circuit is through the column 51 and the flange 1, to which a second connecting lug (not shown) is attached.

There are two other features comprised within the filament-grid structure and their supporting systems. The first of these is a sliding plug 99 mounted in the end of the inner support column 25, and adjustable as to position by means of an operating rod I00, and an offset extension rod l0l passing through a Wilson seal I02 in the gland box 42. The second is a cooling pipe I03 which extends substantially the full length of the inner column 25 and is soldered thereto adjacent its upper end for better heat transfer.

We are now in a position to consider the electrical characteristics of the filament-grid structure in View of the desiderata above set forth, and it is believed appropriate to do this at this point, smce the same principles are involved in the supports for the remaining elements of the device and the explanation of all will be simplified if these principles are in mind. The necessary separation of the elements as regards D. C. or low frequency potentials have already been accounted for. There is no metallic connection between the grid-support column 25 and the filament-support system comprising the column 51, and the support pipes 62. Remaining to be accounted for I the line.

13 is the impedance relationship between the grid and filament members, and this is dependent upon the impedance of the coaxial transmission line formed by the inner and outer columns 25 and S'Iitand the coaxial conductors associated therew h.

The impedance characteristics of transmission lines whose lengths are of the same order of magnitude as the wavelengths of electrical oscillations transmitted thereby are now well known, but they are restated here for convenienc in the explanations that follow. Most of them can be derived from the impedance diagram of a halfwave line open at the output end, as shown in Fig. 32, which indicates such a line diagrammatically, and shows the approximate curve of relative impedance looking into any portion of the line from the right, resistance of the conductors themselves being assumed to approach zero. Extremely short sections show a high capacitive reactance, which falls to the characteristic impedance of of the line at the A; wavelength point, and to zero at the quarter-wave point, i. e., a quarter-wave apen-ended line acts as a dead short. From this point on the apparent reactance is inductive, rising again to the value of at the %7\ point and approaching infinity at V The same diagram may be taken as representing the impedance of, a shorted-end line if the origin be taken at the nodal or quarter-wave point, which appears as a short when looking into For short sections the reactance is small and inductive, it rises to at the /2x point and approaches infinity at V4. Since this appears as an open circuit, increasing the length of the line repeats the P rtion of the diagram shown at the left of the nodal point.

Stated in another manner, a quarter-wave open line or a half-wave shorted line appears much like a series resonant circuit, while a quarter-wave shorted line or a half-wave open line appears like an anti-resonant or parallel resonant circuit.

The only other relationship necessary to the understanding of the present invention is that the characteristic impedance of a quarter-wave line 'is the geometric mean between its input and clos-v From this aspect the first section oithe struc- I ture is the section including the adjusting rods 52, 4!, etc., the flange l0, and the section of the tubular conductor 54 illustrated as below the end of the column 51. Electrically this portion of the structure is a single conductor, and viewed from its upper end constitutes an end-fed antenna. It is preferable that its length be of the order of one-half wavelength at the operating frequency of the device, in which case its effective impedance Z2 will be in the neighborhood of 1,000 ohms. If its length be reduced to one-quarter wavelength its efiective input impedance will likewise be reduced 'tothe neighborhood of from 50 to 100 If we consider the quarter-wave condition to be fulfilled exactly the impedance looking into the coaxial line from the grid end will be If the antenna section of the system has an im- The lines comprising the element supports of the tube of my invention may be considered from a number of aspects, all depending on the general relationships above set forth, but in the treatment here adopted they are generally considered as divided into sections of quarter-wave length, or thereabout, as this is believed to lead to the simplest explanations.

We are interested in the impedance of the gridpedance of the order of 1,000 ohms, the characteristic impedance of the line being 10 ohms, the input impedance of the line will therefore be it: of an ohm. This low impedance therefore becomes the closing impedance of the section of line immediately preceding it. From one point of view it acts as a radio-frequency by-pass between the inner conductor 54 and the outer conductor 51, so that viewed from the input end, at radio-frequencies the cylinder 54 and outer column 51 appear as a single conductor, and form, in connection with inner column 25, a singl radio-frequency transmission line considered as fed from the grid-filament end through a slight impedance irregularity where the inner cylinder 54 terminates. Its efiect from another point of View will be considered later.

Even if the conditions as to impedance of antenna and length of the coaxial lin constituting the column 5'! and cylinder 54 are not exactly met the result will be substantially thesame. The antenna impedance can easily be kept above '100 ohms, making the impedance looking into the quarter wavelength line 1 ohm. If the length of the line section is not exactly one quarter wave, but is still materially greater than oneeighth wavelength, the input impedance will still be low in comparison with the characteristic impedance of the line, and although more power will escape than if optimum conditions are met the amount of power wasted by such undesired radiation will be very small.

The section of the inner line comprising the cylinder 54 and column 25 terminates in the slider 5|, which, as it is of large area and makes good contact with both conductors, may be considered as of zero impedance. This section may be tuned to exactly one-quarter wave by moving the slider.

Due to the spacing between the two conductors the characteristic impedance of this section of transmission line is high, and if the resistance of the line were zero the input impedance would be infinite. Actually it may always be made to exceed 100,000 ohms and under optimum condi- This sec-' tions may reach. ten times this value. tion therefore forms a tuned radio-frequency choke of extremely high impedance interposed between the filament-grid structure and the outside world, and the impedance involved is so high that practically all energy reaching it is reflected back to its source.

What actually happens can be expressed more nearly in the terms of low frequency power line transmission if we think of the antenna as a load which is fed by a line terminating immediately above the top of the column 54. Current fed to this line from the central column 25 must proceed down the column, across the slider, and back to the top of-the conductor 54, since owing to-skin eifect none will flow transversely through the wall of the conductor. In so flowing the current meets an enormous impedance-say 100,000 ohms. From there the line continues down the outside of conductor 54 to the antenna and back within the column 51 to the terminus above the top of conductor 54. This latter length of line, including the load imposed by the antenna, has an impedance of, say 1 ohm, and since the voltage available at the termini of the line will divide itself across this low impedance and the high impedance line section in series therewith in the proportions of the magnitudes of those impedances, andsince the current flowing at the input points of the respective sections is the same, it follows that the energy delivered to the respective impedances will also be in proportion to their magnitudes, and only $5 of that delivered to the line will be transmitted to the antenna to be radiated thereby-still less if optimum conditions are met.

From still a slightly diflerent aspect, the small and largely resistive impedance offered by the outer line is at a current node. We therefore have a very small current flowing, and therefore the value of FR is vanishingly small, the R in this case being the apparent input impedance of the outer line and FR (practically) the energy radiated.

From whatever aspect the matter be considered the result is the same: The sections of the transmission line above the current node terminate in what is equivalent to an open circuit, just as would a low frequency line connected across an ordinarily good insulator. There is some consumption of power, which can be neglected in further consideration, (as in the case of the insulator) and the succeeding sections can be treated as if they terminated at this point in an infinite impedance. It should be noted, however, that at the frequencies we are considering substitution of an insulator for the line sections would drop the impedance to a finite value and introduce large losses through radiation and dielectric phenomena.

' the length of the upper section increases with decreased frequency of operation or vice versa,

2, 16 This eifect is obtained by means of the irregularity introduced by the low-impedance line section constituted by the slider 29. From the top of the high impedance section already described to the slider is a length of relativelyhigh impedance line of less than A wawelength which therefore appears as a capacity variable from zero to some small value as the slider is moved to change its length from zero toward M4. To this is connected the relatively great capacity of the slider portion of the line about ,41 x in length, but presenting an effective capacity many times as great as and apparently in parallel with that of the lower section, so that moving the slider to change the length of the section below it changes the apparent capacity'as viewed from above relatively little. Therefore a very short length of the high impedance line above th slider is all that is necessary to tune this effective capacityto resonance, thus completing the quarter-wave section of line and bringing the node or quarterwave point of the composite section a small distance above the upper slider face. The distance between the slider and the node will vary with frequency, of course, but only slightly with the position of the slider.

It has already been pointed out that the node is effectively equivalent to a short-circuit, and hence, since by moving the slider we may move the position of the node, by so doing we may tune the uppermost section of the line including the filament and grid. We have made thatportion of the line below the slider and above the impedance loop relatively ineffective in tuning, so that we have an "elastic or extensible quarter-wave section of line.

The final or grid-filament section may thus be resonated or otherwise tuned to give optimum operating conditions. In the case of Fig. 1, where capacity feed-back between anode I3 and grid cap 99 is used, the desired tuning of this section must provide a capacitive reactance. This is obtained by making the grid-filament section slightly longer than one-half wavelength or, in other terms, tuning it to a slightly lower frequency than that of the desired oscillation, so that as viewed at grid and filament it presents a small anti-resonant capacitive reactance. Under these circumstances the filament-grid system appears as a capacity in series with the capacity between the grid structure and the anode, and this latter capacity is adjustable by varying the position of the cap 99. When, therefore, the potential of the anode swings, the grid will assume a potential with respect to the filament (and ground) which is intermediate between cathode and anode potential, and which bears the proportion to the total potential between anode and filament that the effective series capacity between anode-grid and grid-filament bears to the apparent capacity between filament and grid. In other words, the arrangement is essentially a capacitive voltage divider which swings the grid potential in the same sense that the anode potential swings, and in fixed and predetermined proportion thereto. Sinc the criterion for oscillation of the device is that the grid and anode should swing in the same sense and in step, the result is a highly effective capacity feed-back which is under control either by varying the actual capacity coupling with the cap 99 why varying the effective resonant input capacity of the grid-filament circuit by varying the position of the slider 29.

By the use of the two sliders the device is thu and thus tunes the filament-grid section. The

actual point ofimportance is that by adjusting the position of the sliders the effective resonant impedance of the filament-grid combination may be made to assume any value which may be desired, since the node above the slider 28 may equally well be made inductive or resistive. Furthermore, since the effective resistances in the circuit are extremely low, and the losses are also small even though the circulating currents may belarge.

' A system of transmission lines, choke and by-passes similar to that used in the filament grid circuit is employed across the filament to prevent transmission of energy to D.-C. insu1a tion and to prevent filament damage by R.-F. currents. The actual ground point on the filament circuit is the flange 'I on the outer casing of the device. This, however, is unimportant and the effect of the'transmission line arrangement may be considered as though the ground point were at the inner end of the filament.

open at its lower end, terminatihg in a high.

impedance. The transmission line impedance is again low, being of the order of, say 5 ohms, and the line therefore forms a negligible series impedance as before, acting as a by-pass to the inner conductor. This, again, is a quarter wave-- length line with the pipe 62 at its inner conductor, terminating in a dead short, and therefore offering very high impedance. As the potential imposed across this impedance is merely that which can build up across the short filament, amounting to a few volts at most, the escape of power through the filament support may be neglected, and the high impedance effectively in series with the filament prevents circulation of R.-F. currents which might otherwise cause hot spots and burn-outs.

We are now ready to consider the mounting of the remaining elements, 1. e., the accelerator grid, boundary grid, and anode, which elements are shown in elevation in Figs. 9, 15 and 13, respectively. The accelerator grid is mounted from a side tube I05. which is welded to project.

through the wall of the housing 5 immediately below the flange 3. This side tube carries at its outer end a flange III'I which is surfaced to receive the tubular glass insulator I09, and the latter, in turn, carries a terminal flange III]. This structure may best be seen in the enlarged detail view of Fig. 6. As in the case of the main tube envelope, the tie-bolts which hold the structure together are omitted, but it will be under-' stood that it is assembled in the same fashion as is the main envelope with ground surfaces reenforced by greased rubber bands or gaskets III which form the seals. Two tubular conductors are fixed to and project inwardly from flange II 0. The inner conductor H2 is spaced from the outer conductor H3 and is held accurately concentric therewith by an annular spacer H4. The accelerator grid I5 is supported from the inner member by a tubular bracket H5, the end of which fits within the conductor I I2 andis rigidly secured thereto. A cooling pipe I", bent into a ring to surround the accelerator grid, has its ends brought down parallel to the support bracket I I5and enters the inner conductor on either side thereof, the ends of the pipe passing into the inter-conductor space distally of thespacer H4 and emerging through the flange H0. A tuning slider H9, which nearly fills the space between the inner and outer conductors and does not 'make actual contact therebetween, is operated by means of a hook I20 whose end projects through a longitudinal slot in the conductor H2. A control rod I2I is threaded to the end of the hook and emerges through a Wilson seal I22.

control grid and the boundary grid through an angular fitting or shield I25,- which passes through a notch I21 cut in one side of the fllament support ring 6 I. This construction is shown in Figs. 1, 6 and 11, each of these figures showin sections of the shield. The shield is electrically continuous with a pan I29 overlying and contacting the filament support ring I4 and slotted immediately above the filaments, which forms an additional shield or barrier to separate completely the anode and control-grid sections of the tube except at the points where intercommunication is necessary or desired. The shield and pan therefore form one terminus, and the accelerator grid and cooling pipe I I1 form the other terminus of the radio-frequency transmission line comprising the side tube I05 and the tubular conductors H2 and H3.

From what has gone before it' is believed that the operation ofthis arrangement will be readily apparent. Again we have an antenna system comprising the control rods I2I and cooling tubes H I, plus the projecting end of the conductor H3, which is fed by and oifers a relatively high impedance to a quarter wavelength transmission line of low impedance formed by the side tube I05 and the conductor H3, and there is accordingly a radio-frequency by-pass between the grounded outer case 5 and the tube I05 of the conductor H3,

- of transmission line comprising the conductors by the spacer H4. This inner line is tuned-to a quarter wavelength by means of the slider I20, which acts as a loading capacity and increases greatly the electrical length of the line. In practice this slider is moved back to a point from which the line appears as a very large inductance at the operating wavelength. The proper point is that at which the remaining inductance and capacitance of the line, considered from the grid end, make it just a quarter wavelength from the inner end. to the shorting spacer H4, forming a very high impedance at the shield where the grid I5 and cooling tube I H are supported, and preventing any appreciable power being transmitted past this point to be radiated.- The capacity of the grid I5 to the boundary grid 4 is large, and that to the control grid I6 is small; there is little coupling tending to swing the accelerator grid I5, and it consequently tends to maintain very nearly zero R.-F. potential.

As has already been described and as shown in detail in'Fig. 16 the boundary grid 4 is firmly clamped between the flange 3 and the anode Within this there is another series section- The boundary rid and the anode face I3 again form the termini of a resonant line, comprising the housing 2 as its outer conductor and a cylindrical anode body, designated by the general reference character I30, within the housing. This i resonant line is one-half wavelength long, and 1 may be considered as terminating between the inner face of the flange I and the end "I of the anode body. This will be recognized as an openended half wave line, and therefore of extremely j high impedance when viewed from either end. The construction and method of support of the l f anode body are best shown in Fig. 8. The sup- 1 port is from the midor quarter wavelength point 1 of the anode, i. e.,-at a potential node, so that l there is little tendency for power to escape from 1 the support structure. I is for power to leak from the support point is suppressed by either or both of two methods. f First, and preferable in the cases where the tube j may be predesigned to operate at a fixed wave- Such tendency as there length, is a movable plate I32 mounted on the sliding rod '50 of the Wilson seal first described, 1 and making contact with the flange I by means :of a spring skirt I33. This may be adjusted to I bring the node of the resonant line accurately at 1 the point of support. This method of preventing direct radiation fromthe anode was adopted in the first of these devices constructed. It was :quickly found, however, that the plate I32 was 1 more useful as a tuning device, and therefore the principle of transmission-lin'e-choke support was %again employed to prevent power escape. In-the 1 construction then adopted and here shown a side ;tube I40 of relatively large diameter is welded at substantially the midpoint of the anode housing 2. The side tube carries a metallic flange I,

1 with a glass insultor tube I42 fitted against it and in turn carrying a terminal flange I43. Through 1this terminal flange passes a pipe I44 which pro- ;jects through a pass hole I45 in the side of the janode housing and on the end of which the anode body is attached. The action here will be deiscribed following the mechanical description of Ithe anode, as the expedients adopted are predijcated upon the necessities of the mechanical structure.

j From the electrical point of view the anode ;body is a simple cylinder with closed ends. Its complexity, as shown in Fig. 8, is due primarily :to the provision for circulating cooling water within it, and to the provision of what may be termed a "rough tuning device.

Owing to the necessity for providing cooling the body itself must be water-tight, and accordingly it is constructed of a -flared cylinder I41, to the flared end of which the anode face I3 is hardsoldered. The other end of the cylinder is closed by a threaded disc I40.

1 The supporting pipe I44 enters the flared cylinder I 4'! through an aperture in the side thereof. The end of the pipe is threaded into a boss I43 on an inner baille cylinder I50, which boss is soldered to the inner wall of the cylinder I41. The boss I48 extends internally to form a cylindrical chamber II, which connects by a side pipe 1I52 through the end I53 of the baflle cylinder, so that water introduced through the pipe I44 is discharged directly against the active face I3 of the anode, and thence is forced around the exterior of the baflle cylinder to reenter its open end. It can then return within the cylinder to 20 is mounted concentrically within the pipe I44 by means of a perforated cap I55 which fits over the end of the pipe I44, its lower -end passing out through the discharge chamber I5I. The cap compresses a rubber gasket I45. sealing the joint between the pipe I44 and the anode body to make it water and vacuum tight. The upper end of the pipe I54 isacentered in the pipe I44 by means of a metal bellow I51 which is sealed to both pipes and permits diflerential expansion between the two. Water is, in-

troduced into the pipe I44 through a side pipe I59, and its course can be traced by the arrowsin the drawings through the outer pipe, the perforations in the cap I55, the side pipe I52, and thence around the baiiie cylinder I50 and back through thecentral pipe I54. 5 A

The action of the mounting follows the principles already set forth, although the application is omewhat different. A disc I is connected to the flange I4I both electrically and mechanically, and carries a cylinder IGI. The pipe I44 and the cylinders I40 and I6I form a transmission line one full wavelength long. Electrically this might equally well be a half wavelength line, but additional space is needed for the insulating cylinder I42, which must withstand the. full D.-'C. anode potential of 20,000 volts or more. The length of this section is measuredfrom the anode and its housing, and the impedance at its outer end i very high, so that looking into it from the anode the impedance is also very high.

This high impedance is connected in shunt across the line formed by the anode body I30 and anode housing 2 very near the nodal. point, where the impedance of the latter line is low, and accordingly a very small portion of the current flowing at this point will take the high impedance path to the outer world.

In other terms, the full wave line is connected so near the node of the main anode oscillator circuit that only a few volts are effective across its termini, and therefore very small currents will tend to flow therein, representing a power loss of V /Z where V is the small input voltage and Z the large input impedance. Moreover[ since the line is one wavelength long, only'the small voltage V will be efiective to cause radi-- ation from the radiating system constituted by the end of the line. It should be noted, however, that by deliberately unbalancing the anode resonator by means of the plate I32 the support system can be converted to a horn antenna which can be made to radiate as much highfrequency energy as the device will produce.

It will be noted that the arrangement described leads to and permits a new typeof cavity resonator; -i. e., one wherein various portions trons are within the resonant cavity. Such acmay be operated at 'diflerent D.-C. potentials, so that D.-C. accelerations can occur while elecceleration is not always necessary or' desirable it is true, but there are occasions where it is useful, and so far as I am aware, none of the various constructions hitherto suggested or employed permit it. It has already been stated that fine tuning of the anode resonatorflcan be accomplished by means of the disc I32. Greater changes in wavelength or operating frequency can be secured by placing a cylindrical shell I62 around the anode body. The effect of such a shell i twofold, since it increases the capacitance of the line formed by the body and the housing, thus tending to inenter the open end of a return pipe I54, which "crease wavelengths or decrease frequency, but it 

